Scheme Note is a Scheme-like music file format and
MIDI conversion tool. It takes a text file, which
is a representation of a song, loosely tied to a
MIDI-like event structure, and outputs a
corresponding MIDI file. It is hand-editable for
simple songs, and should be very easy to parse for
a GUI.

ZynAddSubFX is a powerful real-time, multi-
timbral software synthesizer for Linux. It has
microtonal capabilities, and the instruments it
creates sound like those from professional
keyboards. It includes effects like reverb,
echo, chorus, and phaser.

Cecilia is a graphic user interface for the sound synthesis and sound processing package CSound. Cecilia enables the user to quickly build graphic interfaces with sliders and curves to control CSound intruments. It is also an editor to CSound with syntax highlighting and a built-in reference.

Virtual Sound provides unlimited effects and modulation for sophisticated audio generation, with unique instrument simulation capabilities to craft your own unique soundscapes. All effects are parametrized and their parameters may be controlled through the outputs of other effects, through samples, or through special modulators. Instrument simulation is based on recursively-linked filter networks. Actual instruments can also be simulated through the statistical learning library incorporated in VSound.

Soundmosaic constructs an approximation of one
sound out of small pieces of other sounds. The
soundmosaic algorithm is: split the target file up
into equal-sized segments, or "tiles". For each
tile in the target file, find the closest match in
the source files, and replace the target tile with
the tile from the source files.

SynthTest is a somewhat simple application for OS X that allows you to pick a MIDI source, a MusicDevice AudioUnit, and an Effect or MusicEffect AudioUnit to play with. You can open an edit window for both the synth and effect. If they do not have a custom edit GUI, the generic editor view will be used. SynthTest has a reference implementation of preset support for Audio Units. It is primarily intended to be used by Audio Unit developers for testing purposes (the source is available), although it can be great fun to play with all by itself. It is complete enough to be used for live performances.

Hydrogen is a simple real-time Drum machine/sequencer. It features a graphical user interface based on Qt, a sample-based audio engine, Jack/ALSA/OSS/PortAudio and Export to disk audio drivers, ALSA MIDI input, ability to import/export XML-based song and drumkit files, 192 ticks per pattern, 16 voices with volume, mute, solo, ADSR, and pan capabilities, and import of samples in wave, au, and aiff format.

Javasynth is a Java-based system for creating
(rather experimental) modular audio systems
called sound modules. This allows
synthesizers of some kind to be assembled in
a modular fashion using oscillators, amplifiers,
envelopes, filters, and other components
familiar to those that have worked with
hardware synthesizers. It includes a
separated synth engine and a Swing user
interface for building sound modules.

Open Sound World (OSW) is a scalable, extensible
programming environment that allows musicians,
sound designers, and researchers to process sound
in response to expressive real-time control. It
combines a familiar visual patching paradigm with
solid programming language features such as a
strong type system and hierarchical name spaces.
OSW also includes an intuitive model for
specifying new components using a graphical
interface and high-level C++ expressions, making
it easy to develop and share new music and signal
processing algorithms.